1. Field of the Invention
The present invention relates to radar signal processing systems. More specifically, the present invention relates to phase error correction in a range migration algorithm (RMA) for synthetic aperture radar (SAR) systems.
2. Description of the Related Art
In imaging applications such as ground mapping, a radar system is used to generate a two-dimensional image of a portion of a ground surface in the range and azimuth (cross-range) directions. A large antenna aperture is required in conventional imaging radar systems in order to achieve a narrow beamwidth and, consequently, fine azimuth resolution. Synthetic aperture radar (SAR) systems have been developed as an alternative means for improving azimuth resolution by synthesizing pulse-to-pulse return signals collected by a moving platform with a small antenna. Signal synthesis from many successive locations of the moving platform accomplishes what would otherwise require a larger antenna aperture.
A polar format algorithm (PFA) has been widely used for high-resolution SAR systems. However, the PFA has the shortcomings of limited depth of focus and geometric distortion that increases with the map size. Range migration algorithm (RMA) is one of the most attractive and advanced SAR processing techniques to avoid the problems with the PFA. A difficulty with the RMA, however, is performing efficient phase correction.
Normal SAR data collection requires phase coherence, not only within each pulse for range resolution, but also from pulse to pulse over the collection time needed for azimuth resolution. The platform position affects the pulse-to-pulse phase coherence over the synthetic aperture. Phase error introduced by the inaccuracy of navigation data or undesirable platform motion causes smearing or duplication of the target image. Since motion compensation at the early processing stage based on the navigation data is not sufficient for producing a focused image, it is a common practice to employ data driven autofocus algorithms in high resolution SAR systems in order to maintain phase coherence and achieve good image quality.
Considering the computational efficiency and simplicity, it is desirable to implement the autofocus function after range compression during batch processing as is usually done for the case of PFA. However, for the case of RMA, it is difficult to implement the autofocus function during batch processing because the signal support areas from different targets are not aligned. For this reason, the autofocus function had to be implemented before batch processing starts, at the cost of increased complexity and processing time.
Because of the difficulty in implementing the autofocus function during batch processing, current RMA systems perform the autofocus function during the pulse-to-pulse processing phase using a separate polar format processing algorithm. This approach, however, has the disadvantage of implementation complexity and tighter processing timeline requirements. Furthermore, the increased timeline requirement makes it more difficult, if not impossible, to implement more advanced autofocus techniques.
In a patent application entitled EFFICIENT PHASE CORRECTION SCHEME FOR RANGE MIGRATION ALGORITHM, application Ser. No. 10/060,647, filed Jan. 30, 2002, by K. M. Cho, the teachings of which are incorporated herein by reference, an efficient phase error correction scheme for RMA is disclosed which allows phase correction to be performed during batch processing. This approach requires the image to be oriented in range-azimuth. However, for current and proposed applications, there is a need for images to be formed in a direction other than range and azimuth.
In the general case when the processed image is oriented in an arbitrary direction especially with RMA, phase correction is more complicated and requires additional processing. A currently used method performs phase correction through separate processing in the pulse-to-pulse phase using PFA. There is no known method for phase correction of arbitrarily oriented images in RMA during batch processing.
Hence, a need exists in the art for an improved system or method for efficient phase error correction of arbitrarily oriented images in a range migration algorithm.
The need in the art is addressed by the present invention, a system and method for focusing an image oriented in an arbitrary direction when the collected synthetic aperture radar (SAR) data is processed using range migration algorithm (RMA). In accordance with the teachings of the present invention, first the data is skewed so that the direction of smearing in the image is aligned with one of the spatial frequency axes of the image. In the illustrative embodiment, the smearing is aligned in the vertical direction. This is done through a phase adjustment that was derived from the requirements for proper shift in the spatial frequency domain. Next, the signal support areas from all targets are aligned by proper phase adjustment in the spatial (or image) domain. The common phase error is then estimated and corrected using autofocus algorithms. The remaining steps include reverse shifting and deskew processing for the reversal of the processing performed earlier for the alignment of common phase error.